Separation by crystallization



July 2,1, 1959 R. A. FINDLAY SEPARATION BY cRYsTALLIzATIoN Filed June 2a, 1954 ATTORNEYZ United States Patent SEPARATION BY VCRYSTALLIZATION obert A. Findlay, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application June 28, 1954, Serial No. 439,734

4 Claims. (Cl. 99-199) This invention relates to separation by crystallization. In one of its more specific aspects, the invention relates to improved apparatus for the separation of components of mixtures by fractional crystallization. In another of its more specific aspects, the invention relates to an improved method for sep-arating the components of liquid mixtures by fractional crystallization.

l 895,835 Patented July 21, 1959 I the crystal mass approaching the melting zone. The exact This application is a continuation-in-part of my United instances in which such processes cannot be successfully utilized. Many chemical isomers have similar boiling points and solubilities and cannot be separated satisfactorily by distillation or extraction. Separation by means of fractional crystallization can be satisfactorily utilized in -many cases in making such separations. Fractional crystallization has one great advantage over other methods of separation in that it is the only separation method which Y theoretically offers a pure product in a single stage of operation in systems in which the desired component of p a mixture solidifies at temperatures above which the other components solidify. Thus, whereas distillation and extraction theoretically require infinite stages for a pure product, crystallization in many cases requires only one. This is because of phase equilibrium in distillation and extraction, while by crystallization, substantially pure crystals can be separated from many solutions in one stage, regardless of the liquid composition. Thus, whereas separation by distillation and extraction becomes more difficult as the purity of the product increases, separation by crystallization becomes easier.

Crystallization is thus well suited, not only to the separation of many chemical isomers which can be separated by no other means, but also to the purification of many` compounds which cannot be economically purified b-y other means. Whereas one stage of crystallization theo retically offers a pure product, attainment of this ideal stage has been difficult. Complete removal of occluded impurities without substantial loss in yield is requiredg This invention results in a very close approach to the ideal crystallization stage.

A method of separating a pure component from a mix,

ture has been devised whereby the mixture to be separated is introduced into a heat exchange zone wherein a slurry of crystals is formed and that slurry of crystals is then introduced into an elongated purification chamber through which the crystals are moved as a compact mass. As disclosed by J. Schmidt, Re. 23,810, and by D. L. McKay in application Serial No. 375,850, filed August 24, 1953, the mass of crystals is then moved through the column to a melting zone wherein the crystals are melted. A portion of liquid corresponding to the melt is caused to move countercurrently through at least a portion of the crystal mass so as to displace occluded impurities from mechanism whereby this displaced liquid corresponding to the melt improvesthe purity of the final product is not completely understood. 5 However, it is presently believed that the substantially pure material which is refluxed through at least a portion of the crystal mass displaces the mother liquor from the crystal mass approaching the melting zone and replaces the mother liquor in the interstices of the crystal mass. A high yield of product is obtained since the high melting product refreezes from the reflux stream as it comes in contact with the cold crystal mass moving toward the melting zone. Thus, the portion of the crystal mass which approachesthemelting zone does not contain any appreciable amount of impurities and the resulting product wlhich is removed from the melting zone is of extremely high purity.

In order to make some separations of the constituents of a mixture by fractional crystallization, it is necessary to adjust the temperature of the mixture to one which is far below the solidification point of the pure desired constituent. For example, when para-xylene is separated from a mixture of isomeric alkyl benzenes, it is necessary to cool the mixture to a temperature in the neighborhood of about 57 to about 76 C., depending upon the concentration of the constituents in the mixture, in order to crystallize an optimum amount `of the para-xylene. When the crystals of para-xylene are introduced into a purification chamber wherein they are moved as a compact mass through the length of the chamber to a melt zone in a manner soas to displace a portion of the melt through at least a portion of the crystal mass, considerable difiiculty is encountered. The substantially pure material obtained by melting crystals in the melt zone has, in many instances, such as in the c-ase of para-xylene, a freezing point which is much higher than the temperature at which the crystals must be formed when in admixture with the other liquid components. For example, pure para-xylene has a freezing point of about 13 C. When such a high freezing point material is displaced through at least a portion of a crystal mass which is at a temperature much lower than :the .freezingy point of that material, the relatively pure material freezes in the interstices of the crystal mass and completely closes off further flow of the melt through the compacted crystal mass.` I have devised a method wherebycontinuous operation can be achieved at a high rate without encountering freezing of a solid impervious crystal plug in the purification column. This problem is, to my knowledge, only encountered in a system using a purification tube, such as is described hereinafter. This improvement is particularly important, since it results in a higher rate of recovery of a product of a purity which is higher th-an has been obtained with other types of fractional crystallization systems.

Liquid which is displaced from the crystal mass is removed from the purification chamber through a filter in 'an upstream portion of the chamber. The filter m-ay be either in the wall of the chamber or may be in the form of `a fporous face of the piston used to move the crystals as a compact mass in a downstream direction through lche purification column, or both types of filter can be used. When operating at the relatively low temperatures at which such crystal purification systems are operated, considerable difficulty is encountered in maintaining the filter surface open to the flow of liquid material. l have found that there is a tendency for crystalline material to lbe forced into the pores of the filter, thus materially reducing the filter area through which liquid can be removed from the chamber. I have devised a method whereby continuous operation of such an elongated purification charn.- y l Fber can be operated continuously at a high rate withomtV clogging of the filter area.

The following are objects of the invention.

$95,835 3 4 Ah ebieet af the ihveatien is to provide au improved svsieai ie. ihe'isepafaiiea ef a Pare eeaipeaeat fraai lia: ,Gmup n. P., uid mixtures. Another object of the invention is to pro- C- C- vide an improved process for the separation of a pure component from a liquid mixture. Another object of .the 5 fethyl eylohexlme 10o-3 1263 1- l. Y .-i

' ycloliexane 81.4 6.5 invention 1s to provide a system, me ding `a lter, fer n-Hepiane 98.52 -ms separating crystalline and non-crystalline material in which ZZA'TYlmEthYIPeUtBUe (isooctane) 99e 3 107- 4 M .a .Y i l Nitromethane 101 -29 the filterl is maintained open for the How of liquid there-A p-Dinmnp ii- 'M e". U n 2 Pentan0ne A n I thm-ugh Anith-er. Obie .of the mvnqn. .to MQW@ 2Methy1-2fbntano1.H f 101.8 11.9 a method for freeing a lter of a crystallization system l0 z-nimeihyiperrtane. 89.4 of crystalline material. Another object of the invention 3'11`1lllllPhthlle-- 93: 3 94-5 le Previas, 'a method fer rapidly sevafaliag a eehapeaeal. havihsv a .relatively high selidiealiea Peint fram a inhe- G C B P F P faire Whieh a felaiivelv law euteelie selisiihealiea "p se." Peialherher aheel ef. lhe ihvea'tieh is saprei/isle va .15 Pfeeess Whieh will Perini,t .lhe eealiaiiesi f lisplaeemeat ef Tan... pure material through at least a portion of a compacted Melhvlevelahexafle Cryst OL-hr ellcts of the i-nvgon 2,2,-Tetramethyl butano.. will. he appareil? le t.hase skilled la the anl avea si .ef the accompanying disclosure. Y

Broadlyjspealring, this invention resides in clearing the lter a o iI crystalline material during the operation of a Crystal puriation chamber through which Crystals are e e e s maxed a. .eeairiael mass as is hiere fully deseriheil Gmupn B. P., imp., hereinafter- 'Ijhe filter or ilters are back-washed with .25 C. C. liqhisl f ehleved freni .the pariiiealiea ehamber hvY inter# fi millehllv. .eslahlishihs a. Pressure diilereatial. belweea the ige-g .-.sgdoyynstream and upstream sides oi the ilter or filters. Bmw, 188:10 715.5 Thus., when shelf a; Pressure diherealial isA established, a f suicient arnoiint of the liquid which has been` removed t .4 from` Vthe system flows baci; through the lter, dislodging Group E l. P., 5;.1., Crystalline.. ma .rial Whieh has tended. te'elese effY the.v C' l filter Orlters.. 0-5...-.-. d-

- .77 .58 ar op trac ori e. f Alhehsh as. l have healed.y auf. hefeiaheiefe this la @with 61, 63.5 vennen, isiaaihealaflv. applicable te systems in which thev e les. -108- a Solidilication point ofv` the desired pure component is con-4 56' 5 *915. sielefahlv higher, lhaa the lemperataie .tevhieh the eem- .l Pehehl is, eeelefl ,mihe lishisl mixture te ferm crystals.. Gr' u F B P F P lheeef, this Piseellleeaahe advaiitageehsly utilized ia. P e0." C!" practically any system tov which fractional crystallization 40 isapplicable sc gzasl to increase thereiciency of the process. Orthwxyipm 144 27,1` This invention 1s advantageous when applied to separa- Melia-Well? j 138-8 474r -A e e l i' Para-xylene 138.5 .2 095.111 many QQUIQQUPQDQB SYSPCUIS the CQIQPQDQRS. i e e' olfwlleh haveliaelieallv lhesame hailing Paint and are, i therefore,- difficult to separate by fractional. distillation. A N l 0f., i9 hlillihfesvhieh have divefselieiliag nein'tsfbutvvhieh GluG l'" form a*zccgxtropres or are heat sensitive. Thereffective s epaf alieh ef.. saai sheets. et. sash mixtures may. be made @,thwymgg; l 175.0 73.5 fem systems vvheie, the concentraties of one component Mammals-Les-;.-.amat:.-;. 115.1A -z5 is rela tive lyhigh, orl wherelthe c oncentration-sofl the com; Payrfffi 175:0 "7315i PQ-Iit-.s 591%? eqlll olle Particular. eldvalltageQuS application ofthe process lies in the purification of acol- Pfflehl 0i, say l5. l0.. 25 pereehi purity., seas iQ ,elfeet a Gump H B" 15): Pllil!.upwadsellpeieeallas .erle illustrate seme. of 'filesystems toVwlhihhtlie` inven uorifisf applicable, the 55 Dimethyiphthala 282 5-5 felleviihs eeiapehaels. are grouped with respect to their. Diaethvliseahthelalesl 124 e? bolllllg POlIlfS Dimethyi terepiithsiate 140.6

roupA B P FSE., n. i C C., 60 Gioupl iisg., Bw gg' i hi www M-.-

11.6.6 n. Avana" .2 l 1;.Hpm1v1`e;. 95- 52l 90,5 i Ortho mtmtoluene y :4 1 garblortatachloride -22-82 Meta 'trotoiuene 231` 1.5.5 or o r ee 5. Ethiaicohol 78.5 117.3' 55 P- oiqm-I-lel 238- 1l 3- -gmegvlpegen gg -125 in@y y D( an -..1i Methyl ethyl keton 79.6 -Bui 1 f-.an.y .of two. or'mQi-e- Methyl piopiona. 19,9 -s7.5 ofthe components within anyl oneofthe grouplsinay vbe Methyl acrylate" 80'5 scpar ed'bYhe v'ln'ocess of the invention, aswell as sys- 1,3Cyc1ohexad1en 80,*5 98M f ,Y .A l. 2,4-D1m'ethy1, en. 80.8y -123.470 tems made up of.- components selected from difterent- @3i-hazte. t 22.5% 122i awesome; beaaeaeaay be. aiaaeaiffom Acetoniniie-- s2 42j Aiiexaiiefor; n -Qlieptane system.ii.1 vvll1i^cl1the ben; Siliiieiv .seaifiaaaaiaearil seeatehlhaalheeateeheeea: a-Meshslhel sal. -ilaqi capitalism Inf lhefsaihemahnef. para-Xyleae, may. he...

,from para, meta-, and ortho-xylenes. vBenzene may also be separated from a mixture with toluene and/or aniline. Multicomponent systems which may be effectively separated so as to recover one or more of the components in substantially pure form include 2,2-dimethylpentane, 2,4 dimethylpentane, 2,2,3 trimethylbutane, methyl cyclohexane, 2,2,4-trimethylpentane, and carbon tetrachloride, chloroform, and acetone. The invention is also applicable to the separation of individual components from a` system of cymenes and a system including the xylenes.

This invention can also be utilized to purify naphthalene, hydroquinone (1,4-benzenediol), paracresol, paradichlorobenzene, and such` materials as high melting waxes, fatty acids, and high molecular weight normal parafiins. The system can also be used to separate anthracene, phenanthrene, and carbazole. Furthermore, the system can be used to separate durene (1,2,4,5tetramethylbenzene) from C aromatics. In cases where the material to be purified has a relatively high crystallization point, the impure material is raised to a temperature at which only a portion of the constituents are in a crystalline state and the resulting slurry is handled at such a temperature that operation is as described in connection with materials which crystallize at lower temperatures.

The :invention is also applicable to the concentration of food products. In the preparation lof such concentrated foods, the p-rocess consists generally of the removal of water from such products. One special class of foods which can be concentrated in this manner is that of fruit juices, such as grape, pineapple, watermelon, apple, orange, lemon, lime, tangerine, grapefruit, and the like. Beverages, such as milk, wine, beer, coffee, tea, and various liquors, can also be concentrated in such a process. The process is also applicable to the concentration of vegetable juices.

A more complete understanding of this invention will be obtained on study of the accompanying drawings, in which:

Figure l is a schematic representation of the fractional crystallization system of this invention; and

Figure 2 is a section view of a modified system of this invention.

Referring particularly to Figure l of the drawings, a mixture of materials from which at least one constituent is to be separated is fed through conduit 11 to a heat exchanger, such as chiller 12, where the temperature of the mixture is `adjusted so as to obtain crystals of at least a portion of at least oneof the constituents of the mixture. Chiller 12 can be any conventional type chiller, preferably being of the scraped surface type which is supplied with refrigeration means that are adequate to lower the temperature of the -liquid mixture to that necessary to crystallize at least a portion of at least one of the constituents thereof. As will be apparent from the discussion hereinbefore, the temperature to which the mixture is adjusted in the heat exchanger will depend entirely upon the specific mixture, since the various materials solidify at different temperatures and the temperatures at which any -given constituent of a mixture will solidify is dependent upon the constitution of the mixture. When a mixture of isomeric C8 alkyl benzenes is fed through conduit 11 to chiller 12, that mixture is lowered to a temperature in the neighborhood of -57 to 76 C. When the desired product is relatively dilute in the feed stream, the range of temperatures at which crystals will form will be relatively low. If the desired product is relatively concentrated in the feed, crystals thereof will form at relatively high temperatures. When materials having a high crystallization temperature are handled, the heat exchanger may be a heater rather than a chiller.

VIf desired, crystals can be formed by introducing a low boiling liquefied refrigerant into the heat exchange chamber, together with the liquid mixture from which component is to be separated, and vaporizing the refrigerant, thereby causing cooling of the liquid mixture with concomitant crystallization of thehigh Vfreezing point component of the mixture. The vaporized refrigerant can then be removed directly to a compression zone where it is reliqueiied by compression and subsequently" cooled.

The slurry resulting from chilling of the liquid mixture is fed through conduit 13 to filter 14 wherein uncrystallized material isA separated from the crystals through conduit 15. The amount of uncrystallized material (mother liquor) which is left on the crystals is dependent upon the specific type of filter which is utilized. Any one of several different types of filters can be used for this step in any process. However, in order to obtain the highest possible etliciency, I prefer to-use a piston type filter-purification chamber, such as the one more fully described hereinafter. The combination of two such filter-purification chambers is disclosed by D. I.-

Quigg in U.S. application Serial No. 443,512, tiled July 15, 1954, and by I. A. Weedman in U.S. application Serial No. 462,828, filed Oct. 18, 1954. By using such a combination, modied as described hereinafter, I am able to maintain all of the filter area of a system substantially free of crystalline material. Crystals are removed from filter 14 through conduit 16 to a heating tank 17. The crystals are heated to such an extent that a portion of the crystals is melted and, in that manner, the crystal mass is diluted and warmed. It is preferred to melt a suflicient amount of the crystals to form a mixture which has a solids content of not more than 45 percent by weight. It is preferred that the material from chamber 17 have a solids content of 35 to 45 percent by weight. In such an operation, the resulting mixture is removed from heater 17 and is passed through conduits 18, 19 and 21 into the upstream portion of purification chamber 22. In another modification, the crystals are heated to a higher temperature in heater 17 so as to reduce the solids content to not greater than 10 per cent by weight. Operating in this latter manner facilitates better conrol of the temperature at which crystals are eventually introduced into purification chamber 22. When the solids content of the mixture is lowered below 35 percent by weight, the material is passed by means of conduit 18 to Chiller 23 wherein the mixture lis cooled so as to form a mixture having a solids content of between 35 and 45 percent by weight.

As the crystals are introduced into chamber 22, piston 24 is actuated by the flow of motivating fluid through conduits 25 and 26, acting on motivating piston 27 connected to piston 24 by piston rod 28 so as to rerciprocate piston 24 in chamber 22. In this fashion, the crys-V tals are moved as a compact mass in a downstream direction toward heating element 29. Heating element 29 may be in the form of heating coils within chamber 22 or may be in the form of a heating element closely surrounding the lower portion of chamber 22 or may be provided in the bottom surface of that column, or may be of any other conventional heater design.

A lter 31 is provided in the upstream portion of chamber 22. This filter is designed so as to permit the removal of liquid from the crystal mass by displacement and by compaction. Piston 24 can be imperforate, in which case all of the liquid displaced from the crystal mass 4is removed from chamber 22 through lter 31. As an alternative, however, piston 24 can be provided with a perforate downstream face, which perforate face permits the passage of liquid material therethrough while moving the -crystals in a downstream direction. When piston 24 is of the perforate type, liquid which is removed through that piston is removed from chamber 22 through conduit means, such as conduit 32. Puri" lied product is removed from the downstream end portion of melting section of chamber 22 through conduit 33. Although the purified product is preferably removed in a meited form, that product can be removed at least Partially es a Se1ii.i,.f desired Liquid material which is removed from chamber 22 31 is removed through conduit 34. The disposition which I make of the liquid material removed through filter 31 or conduit 32 depends to a ilarge extent upon the amount of crystals melted while bringing the crystal mass to a relatively warm temperature in heating chamber 17 or the concentration of the desired component in the liquid removed from chamber 22 through the filters. it ispreferred tooperate the system so that the liquid Whieii. is reigeved through filter 31 er through @enduit 32, is euriehed in, the eempenent which is being purified chamber 2 2, compared to the feed introduced iiii'iiell @enduit 1L Beeause the liquid is enriched in that component, that liquid is suitable for return to chiller 12 together with the feed mixture supplied through conduit 11. The liquid is returned through conduits 34 and 3$. In this fashion, larger crystals of the desired product are formed in the chiller, which larger crystals are more easily processed in this system. When the concentration of the desired component in the liquid removed from chamber 22 is not higher than that in a mixture fed through conduit 11, that liquid is not sufficiently enriched in the desired product to benefit the feed. In such a situation, that liquid is usually removed thtrough conduit 36 -and/or conduit 32 and can be discarded, or in the case of some mixtures, such as the isomeric alkyl benzenesv, it can be isomerized to produce additional desired products.

In one aspect of my invention, I produce intermittently a'presisure differential, generally in the range of from 1 to 7 p.s.i., across the filter in filter 31 and in piston 24, by maintaining a back pressure on the liquid being removed through conduits 34 and/or 32. Such a back pressure is conveniently maintained by means of valves 37, 38 and 39. A less iiexible way of maintaining a backl pressure is by introducing the liquid into the lower portion of a storage tank in which a considerable head of liquid is maintained. Thus, in a piston type chamber, as piston 24 is moved in an upstream direction, pressure is reduced in lthe area between the upstream end of the compact crystal mass and the downstream end of piston 24. Liquid is caused to flow back through the filter by reason of the resulting pressure d ierential, thereby removing crystalline material' from the filter. Y

Referring particularly to Figure 2 of the drawings, parts like those described in connection with Figure l are designated by likel numerals. In this modification, pulsation chamber 4.1 is connected to conduit 34. This chamber is provided with a piston 42 which is slideably positioned in that chamber, being adapted to slide within chamber 41 while preventing the 'passage of liquid material around or through that piston. Chamber 41 is connected at one end portion to conduit 34 while the other end portion of that chamber is connected by means of conduit 44 to conduit 32. Liquid which is removed from chamber 22 during that movement of piston 24 in its downstream direction is passed through conduit 34. The movement of' piston 24 in its downstream direction causes the liquid to flow through conduit 34 under such pressure as to move piston 42 -in a direction away from conduit 34 and to fill the lower portion of that chamber with. such liquid. As piston 24 moves in an upstream direction, liquid which has been displaced through piston 24 is moved through conduit 32 anda portion thereof back-washes the filter in the face of piston 24 and also causes piston 42to move toward conduit 34, thus causing4 a surge ofl liquid in conduit 34 so as to provide an impulse of liquid to assist in back-washing filter 31. y

'In order to better illustrate my invention, I have prof videdtllefollowing example which exemplifies this. inyeg...

tion but should not be construed to unduly limit the same.

Example A feed mixture comprising 17 percent para-xylene, 17.5 percent ortho-xylene, 33.4 percent meta-xylene, 27.5 percent ethyl benzene and 4.6 percent toluene is supplied to the system at a rate of 1000 gallons per hour through conduit 11. Liquid material recovered as displaced. liquid from chamber 22 through filter 31 comprises 53.1 percent para-xylene, 9.8 percent ortho-yylene, 18.7 per cent meta-xylene, 15.6 percent ethyl benzene and 2.8 percent toluene. A back pressure is maintained on the liquid material recovered from chamber 22 through filter 31 andv conduit 34. This stream is supplied to Chiller 12 at 165 gallons per hour together with the feed stream describedl hereinabove. The resulting composite feed comprises1 22.1 percent para-Xylene, 16.3 percent ortho-xylene, l3l' percent meta-xylene, 26.2 percent ethyl benzene and 4.4 percent toluene at 1165 gallons per hour. That material is lowered to a temperature of -76 C. with the resultant formation of- 15.1 percent solids. The slurry of mother liquor and crystals is passed to filter 14 wherein mother liquor having a para-xylene content of 6.75 percent is removed through conduit 15 at a rate of 890 gallons per hour. The crystalline material from which the mother' liquor has been removed is percent solids and is at. a temperature of -76 C. That material is supplied to heating chamber 17 wherein the crystal mass is heated to a temperature of -239 C. and a stream of 40 percent solids at that temperature is moved through conduits 18.,

19 and 21 to purification chamber 22 at a rate of 275 gallons perhour. The crystals are moved downstream through chamber 22 as a compact mass by means of the intermittent strokes of piston 24. As the crystals are moved to the heating zone, the crystals are melted and a kportion of the melt is displaced upstream through the compact crystal mass. A product which is about 99 percent para-xylene is removed through conduit 33 at a rate of lgallons per hour. As piston 24 is moved in an upstream direction, a zone of lower pressure is created between the upstream end of the compact crystal mass and the face of the piston so that a pressure differential exists across filter 31, e.g., a pressure dierential of 5 p.s.i. and liquid in conduit 34 is caused to flow back through that filter, thereby washing the filter substantially free of crystalline material.

It will be apparent to those skilled in the art that 'various modifications of this invention can be made upon study offY the. accompanyingy disclosure. Such modifications are believedto be clearly within the spirit and the scope of this invention.

I claim:

1. A process for purifying an impure crystalline material, which comprises intermittently moving an'elongated compact mass of impure crystals through an elongated confined displacement zone toward a melting zone at one end thereof by the reciprocating movement of a piston spaced upstream from said melting zone; melting crystals in said melting zone; displacing a portion of resulting melt countercurrently through at least a portion of said compact mass of impure crystals so as to displace liquid therefrom; removing displaced liquid from an upstream portion of said displacement zone through filter means; maintaining a back pressure on said displaced liquid so removedv so as to cause a back iiow of said displaced liquid through said filter means -upon the back stroke of said piston so as to dislodge any crystalline material from said filter means; and removing purified productl from said melt zone.

2. A process for purifying an impure crystalline material, which comprises intermittently moving an elongated compact mass of impure crystals through an elongated; confined displacement zone toward amelting zone at one enti. thereof. by the. reciprocating. movement of. a` einenV spaced upstream from said melting zone; melting crystals in said melting zone; displacing a portion of resulting melt countercurrently through at least a portion of said compact mass of impure crystals so as to displace liquid therefrom; removing displaced liquid from said crystal mass through lter means in the wall of said displacement zone and lter means in the face of said piston; collecting a portion of said displaced liquid removed through the iilter means in the wall of said displacement zone in a reservoir; rapidly ejecting said liquid collected in said reservoir in response to pressure applied by said piston to the liquid removed through said piston so as to apply a surge of said displaced liquid back through said filter means so as to dislodge any crystalline material there- 'om; and removing puried product from said melt zone.

3. A process according to claim 2 wherein said displaced liquid is forced back through said filter means under a differential pressure in the range 1 to 7 p.s.i.

4. Apparatus for crystal purification which comprises a tube; a melting chamber in communication with one end of said tube; heating means operatively connected to said melting chamber; a product outlet from said melting chamber; a crystal inlet spaced upstream from said melting chamber; piston means in said tube adapted to reciprocate therein so as to move said crystals as a compact mass through said tube toward said melting chamber; rst lter means in the face of said piston;

second lter means in the wall of said tube, spaced upstream from said melting chamber; rst conduit means extending from said filter means in the wall of said tube; a reservoir chamber connected to said first conduit means; piston means fitted in said reservoir chamber so as to reciprocate therein but so as to prevent the flow of liquid around said piston; second conduit means connected to said tube upstream of said lter in said piston and connected to said reservoir chamber at the end opposite that connected to said first conduit so that liquid moved through said second conduit means acts against one side of said piston and liquid moved through said first conduit means acts against the other side of said piston.

References Cited in the le of this patent UNITED STATES PATENTS 2,239,612 Lawlor Apr. 22, 1941 2,337,385 Gross Dec. 21, 1943 2,475,968 Jokel July 12, 1949 2,603,667 Pankratz et al. July 15, 1952 2,613,136 McKay Oct. 7, 1952 2,615,793 Weedman Oct. 28, 1952 2,622,115 Carney Dec. 16, 1952 2,683,178 Findlay July 6, 1954 2,731,456 Weedman Jan. 17, 1956 2,747,001 Weedman May 22, 1956 2,752,230 Findlay June 26, 1956 

1. A PROCESS FOR PURIFYING AN IMPURE CRYSTALLINE MATERIAL, WHICH COMPRISES INTERMITTENTLY MOVING AN ELONGATED COMPACT MASS OF IMPURE CRYSTALS THROUGH AN ELONGATED CONFINED DISPLACEMENT ZONE TOWARD A MELTING ZONE AT ONE END THEREOF BY THE RECIPROCATING MOVEMENT OF A PISTON SPACED UPSTREAM FROM SAID MELTING ZONE; MELTING CRYSTALS IN SAID MELTING ZONE; DISPLACING A PORTION OF RESULTING MELT COUNTERCURRENTLY THROUGH AT LEAST A PORTION OF SAID COMPACT MASS OF IMPURE CRYSTALS SO AS TO DISPLACE LIQUID THEREFROM; REMOVING DISPLACED LIQUID FROM AN UPSTREAM PORTION OF SAID DISPLACEMENT ZONE THROUGH FILTER MEANS; MAINTAINING A BACK PRESSURE ON SAID DISPLACED LIQUID SO REMOVED SO AS TO CAUSE A BACK FLOW OF SAID DISPLACED LIQUID THROUGH SAID FILTER MEANS UPON THE BACK STROKE OF SAID PISTON SO AS TO DISLODGE ANY CRYSTALLINE MATERIAL FROM SAID FILTER MEANS; AND REMOVING PURIFIED PRODUCT FROM SAID MELT ZONE. 